Roux-en-Y gastric bypass surgery (RYGB) causes profound, long-term weight loss and dramatic remission of type 2 diabetes in patients, and recent evidence suggests that this operation works primarily by altering metabolic physiology. RYGB induces changes in the secretion of several gastrointestinal (GI) hormones in patterns that suggest their contribution to metabolic outcomes after surgery. Among these hormones, glucagon-like peptide-1 (GLP-1), an incretin hormone that promotes insulin release and inhibits food intake, is particularly attractive as a mediator of the improved glucose homeostasis and weight loss effects of RYGB. This proposal aims to define the contribution of GLP-1 signaling in mediating the improved glucose homeostasis and energy balance seen after bypass surgery, using a mouse model of RYGB that closely mimics the human operation. In Aim 1, the extent to which global GLP-1 signaling is involved in mediating specific metabolic outcomes of RYGB will be determined. A genetic mouse model will be employed, in which a floxed GLP-1 receptor gene is deleted in adulthood by global but inducible activation of a Cre recombinase. This model circumvents the possible confounding effects of GLP-1R deficiency during early development. RYGB surgery in this whole-body GLP-1R deficient model will be performed, and mice will be carefully characterized with respect to glucose homeostasis and energy balance. Parameters including glucose and insulin tolerance, glucose stimulated insulin secretion, body weight and composition, food intake, and energy expenditure, will be assessed to determine the role(s) of global GLP-1 signaling in mediating RYGB outcomes. In Aim 2, potential critical sites of GLP-1 signaling in mediating selective physiological effects of RYGB will be defined. Using targeted pharmacological inhibition of GLP-1R activity, the contribution of central GLP-1R to RYGB outcomes will be determined. Mice with RYGB will be chronically administered the GLP-1R antagonist Exendin(9-39) directly into the brain via an intracerebroventricular cannula connected to a mini-osmotic pump. Next, using genetic approaches, the contributions of GLP-1 signaling specifically in hypothalamic neurons and pancreatic ?-cells will be performed using mice selectively expressing Cre in these sites. The Cre recombinase should remove the floxed GLP-1R in the targeted tissues, allowing for the site-specific determination of the GLP-1 function in glucose homeostasis and energy balance outcomes after RYGB. Identification of the precise role and location of action of GLP-1 signaling in the mediation of RYGB outcomes will help facilitate the development of novel, effective but less invasive therapies that will be applicable o a much broader segment of the population with obesity and type 2 diabetes. These studies will also provide important information to enhance our understanding of the role and mechanisms of GI regulators of metabolic function more broadly.